Router: client IP address replacement.

This commit introduces the replacement of the client address based on the value
of a specified HTTP header.  This is intended for use when Unit is placed
behind a reverse proxy like nginx or a CDN.

You must specify the source addresses of the trusted proxies.  This can be
accomplished with any valid IP pattern supported by Unit's match block:

["10.0.0.1", "10.4.0.0/16", "!192.168.1.1"]

The feature is configured per listener.

The client address replacement functionality only operates when there is a
source IP match and the specified header is present.  Typically this would be
an 'X-Forwarded-For' header.

{
    "listeners": {
        "127.0.0.1:8080": {
            "client_ip": {
                "header": "X-Forwarded-For",
                "source": [
                    "10.0.0.0/8"
                ]
            },
            "pass": "applications/my_app"
        },
    }
}

If a request occurs and Unit receives a header like below:

"X-Forwarded-For: 84.123.23.23"

By default, Unit trusts the last rightmost IP in the header, so REMOTE_ADDR
will be set to 84.123.23.23 if the connection originated from 10.0.0.0/8.

If Unit runs behind consecutive reverse proxies and receives a header similar
to the following:

"X-Forwarded-For: 84.123.23.23, 10.0.0.254"

You will need to enable "recursive" checking, which walks the header from
last address to first and chooses the first non-trusted address it finds.

{
    "listeners": {
        "127.0.0.1:8080": {
            "client_ip": {
                "header": "X-Forwarded-For",
                "source": [
                    "10.0.0.0/8"
                ]
                "recursive": true,
            },
            "pass": "applications/my_app"
        },
    }
}

If a connection from 10.0.0.0/8 occurs, the chain is walked.  Here, 10.0.0.254
is also a trusted address so the client address will be replaced with
84.123.23.23.

If all IP addresses in the header are trusted, the client address is set to
the first address in the header:

If 10.0.0.0/8 is trusted and "X-Forwarded-For: 10.0.0.3, 10.0.0.2, 10.0.0.1",
the client address will be replaced with 10.0.0.3.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/nxt_router.h b/src/nxt_router.h
index 6611cf45..fc068b53 100644
--- a/src/nxt_router.h
+++ b/src/nxt_router.h
@@ -18,6 +18,7 @@ typedef struct nxt_http_request_s  nxt_http_request_t;
 
 typedef struct nxt_http_action_s        nxt_http_action_t;
 typedef struct nxt_http_routes_s        nxt_http_routes_t;
+typedef struct nxt_http_client_ip_s     nxt_http_client_ip_t;
 typedef struct nxt_upstream_s           nxt_upstream_t;
 typedef struct nxt_upstreams_s          nxt_upstreams_t;
 typedef struct nxt_router_access_log_s  nxt_router_access_log_t;
@@ -195,6 +196,8 @@ typedef struct {
 
     uint8_t                discard_unsafe_fields;  /* 1 bit */
 
+    nxt_http_client_ip_t   *client_ip;
+
 #if (NXT_TLS)
     nxt_tls_conf_t         *tls;
 #endif